The invention disclosed and claimed herein deals with compositions that control the formation of crystalline hydrates in various systems, most notably, gas and oil transmission pipeline systems. The compositions are comprised of carbon dioxide sorbing polymers that also have the capability of driving the formation of hydrate crystals into the polymeric matrix.
Crystalline hydrates can form in oil and gas pipelines carrying oil and gas if the chemical composition of the produced fluids includes water, either or both of ethane or methane with carbon dioxide and sometimes, other hydrocarbon gases and/or sulfur dioxide. In addition to the chemical composition, there is a need for a driving force for such hydrate nucleation involving the physical and environmental conditions.
The fluid composition generally is at an elevated temperature, typically above 70° C. and it will cool to a lower temperature, typically below 16° C., whereby the gases and water become super saturated and crystallize from solution at the lower temperature. The pipeline linking the sub-sea oil producing well to the processing platform is the crucial environment for the formation of the hydrate crystals. The surrounding seawater with temperatures that are about 4° C. to about 6° C. cools the pipeline that is carrying the produced fluids and obviously, the oil or gas contained therein.
Many years ago, when fluids were produced directly onto the production platform and arrived there at high temperatures, hydrates did not occur since the temperature of hydrate formation was typically between 15° C. and 22° C. When the fluids arrived on board the processing plant at 25° C. to 40° C. the hydrate formation was not an issue. But, as oilfields became larger and more diverse and the use of sub-sea producing wells became the normal practice, fluids produced therefrom would be cooled by the 4° C. to 6° C. seawater around the sub-sea pipeline. Such situations were optimal for crystal hydrate formation.
One of the main reasons for dealing with crystalline hydrates at all stems from the fact that the hydrates are mixtures of water, methane, ethane, carbon dioxide and sulfur dioxides, and these materials combine under ideal pressure and temperature conditions. These solid hydrates, once formed, will grow in size to eventually plug flow lines, and in some instances, once formed, travel through the pipelines at such velocities that they become dangerous projectiles having high potential to puncture holes in the pipe line.
Thus, the formation of hydrates is a costly and potentially environmental challenge for the oil and gas industry. As global offshore deepwater mining of oil and gas increases, the challenges of preventing or diminishing the formation of hydrates remains the industry technical challenge.
Prior art methods for controlling the formation of hydrate crystals in pipelines include the continuous injection of methanol, ethanol, or glycol; offshore dehydration of the gas so produced; warming fluids under normal flow conditions through insulation; heating flow lines, and using low doses of chemical inhibitors for threshold hydrate inhibition, kinetic inhibitor polymers, surfactants and emulsions, and anti-agglomerate polymers and surfactants.
Of these, methanol, ethanol and glycol are currently practiced but the environmental and financial costs are high. Methanol and glycols are added to pipeline fluids at about 30 to 50% by weight of water co-produced. The costs are high but the logistics for supply and storage offshore and more importantly pumping to sub-sea producer wells are significant and cumbersome.
Offshore dehydration is not feasible for production from sub-sea producing wells and the strategic option for warming the pipeline by heated water or other fluids from the processing platform requires double wall pipeline, that is both expensive and logistically, a difficult operational process. Heating the flow lines when sub-sea is also expensive and logistically problematic and flawed with respect to reliability. The low dose chemical inhibitor is the new area and is currently under examination by a range of chemical suppliers trying to develop low cost and high performance inhibitors
The threshold and kinetic inhibitors function to prevent the growth of hydrate crystals and act essentially like salt acts to depress the freezing point of water. However, the tolerance is not as great as the industry requires and they fail in the majority of demanding situations. These are typically surfactants and polymers such as polyvinyl pyrrolidone or polyvinyl pyridine, or polyvinyl caprolactum.